1. Field of the Invention
The invention relates to a local communication system for digital serial data and various apparatus for use as transmitting and/or stations of such a network. The invention has many applications but is particularly intended to provide a low cost network for the integrated distribution of digital audio signals or other, high-volume xe2x80x9csource dataxe2x80x9d, together with control messages, in vehicles.
2. Related Art
A local communication system which combines source data (CD audio, MPEG video, telephone audio, navigation data etc) with control commands in a low cost optical fibre network is available in the form of the D2B Optical system. For details, see for example the xe2x80x9cConan Technology Brochurexe2x80x9d and the xe2x80x9cConan IC Data Sheetxe2x80x9d available from Communication and Control Electronics Limited, Stirling House, Stirling Road, The Surrey Research Park, Guildford, Surrey GU2 5RF, United Kingdom (http://www.candc.co.uk). See also German patent applications of Becker GmbH, Karlsbad, Germany with filing numbers 19503206.3 (95P03), 19503207.1 (95P04), 19503209.8 (95P05), 19503210.1 (95P06), 19503212.8 (95P07), 19503213.6 (95P08), 19503214.4 (95P09) and 19503215.2 (95P 10). xe2x80x9cConanxe2x80x9d is a registered trade mark of Communication and Control Electronics Limited. The gross data rate in the D2B Optical system is 5.64 megabits per second (Mbps) (for an audio sampling rate of 44.1 kHz). Each segment of the network can carry 4.2 Mbps xe2x80x9csource dataxe2x80x9d (for example three xe2x80x9cCD qualityxe2x80x9d stereo audio channels of 1.4 Mbps each), together with control messages at a gross rate of about 176 kbps.
Although low cost fibre and electro-optical components can be used in the D2B Optical system, nevertheless the cost of these is a significant part of the cost of the network, and the range of commercially viable applications could in principle be widened by adopting, for example, a simple wire cable, if problems of susceptibility to and generation of spurious electromagnetic radiation and noise signals (EMI) could be eliminated.
Another proposal for an in-car network is CARNET, using a chip COM20023 available from Standard Microsystems Corporation, Hauppauge, N.Y. (see http://www.smc.com). CARNET uses ARCNET network protocols (ANSI Standard 878.1), with additional frequency shift keyed (FSK) encoding, which may be intended to permit wired networks with low EMI noise emission. The data rate with FSK is 5 Mbps, comparable to D2B Optical. The CARNET chip also provides an interface to CD-ROM for accessing navigation databases, and this is said to be suitable for any standard audio datastreams also.
However, the CARNET chip generates the FSK waveform by digital synthesis, imposing high frequency quantisation noise on the desired band-limited signal. In practice, this noise includes components in the FM radio frequency band (87-108 MHz), and filtering to suppress this adequately in an in-car audio system is difficult and expensive. The COM20023 data sheet proposes a low pass filter including an inductor, but with no indication of the cost or efficiency of this solution in terms of FM radio interference. Also, since the digital to analogue converter (DAC) for the FSK waveform is external to the CARNET chip, radiation of high frequency components will inevitably occur from the DAC and circuit wiring before the filter.
FSK can be regarded as a binary version of frequency modulation (see J Dunlop and D G Smith, xe2x80x9cTelecommunications Engineeringxe2x80x9d (Second Edition), Chapman and Hall, 1989, ISBN 0-412381907). Two carrier waveforms of different frequencies are defined and switched between, depending on the data bit values. A further form of modulation offering a more confined spectrum than FSK is phase shift keying (PSK), known and used in modems for lower-rate data communications via voice channels. In PSK different phase components at a single carrier frequency are selectively switched between, depending on the data bits.
In a standard 9600 bps modem, however, the basic binary PSK scheme is elaborated so that quadrature phase modulation and amplitude modulation are combined with a baud rate (symbol rate) of 2400 baud, to achieve a data rate of four bits/band.
Such techniques require complex adaptive equalisation and coherent decoder circuits, however, and other media such as optical fibre or coaxial cable are conventionally used for wide band local area networks of the type presently desired.
The invention provides an alternative form of network based on analogue PSK modulation circuitry, and novel circuits for transmitting and receiving digital data by PSK modulation, as defined in the appended claims. The invention provides in effect a high frequency modem having very low noise emissions, compared with comparable known designs.
In one embodiment, the PSK encoding and decoding can be implemented by a small adaptor circuit connected between a digital communication interface (for example the CONAN IC) and its timing crystal. Such an interface might for example be a Conan IC implementing D2B Optical protocols, an ARCNET interface, or some other interface.
A novel PSK decoder forming part of the high frequency modem comprises a simple integrator circuit (low pass filter), followed for example by a comparator.